Is your Wal-Mart making you fat?

McDonald's often gets some blame when people jaw about the national obesity rate. Now new research by economists indicates that Wal-Mart is a factor in weight gain -- possibly because of low prices for unhealthful processed foods.

The study abstract says:

An additional Supercenter per 100,000 residents increases average BMI by 0.24 units and the obesity rate by 2.3% points. These results imply that the proliferation of Walmart Supercenters explains 10.5% of the rise in obesity since the late 1980s, but the resulting increase in medical expenditures offsets only a small portion of consumers' savings from shopping at Supercenters.

The study focused on the period between 1996 and 2005, when 1,569 supercenters opened their doors across the U.S.

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The study doesn't suggest that we'll all end up using the motorized carts once a Wal-Mart supercenter opens nearby. It did find that "women, low-income families and people living in less densely populated areas" are most prone to pack on pounds after the supercenter arrives, The Gazette of Montreal reported. And that's after other possible causes were accounted for.

Why would this happen? After all, those supercenters provide a large supply of fresh fruits and vegetables and sell them at a competitive price.

In short, the cause is unknown. The study authors speculated that it could be the

other groceries Wal-Mart sells -- the processed foods that fill most of the shelves. Post continues after video.

Also: "It's not just about Wal-Mart underselling the competitor. It's about the competitors cutting their prices in response to competition from Wal-Mart," University of North Carolina economist Charles Courtemanche, a co-author of the study, told The Gazette. "Someone might never step foot in a Wal-Mart, but they still might pay less for their food."

Maybe it's also the bigger selection of video games, which translates into less exercise.

Mother Nature Network blogger Siel Ju raised another possibility:

Many experts have said that having a Wal-Mart pop up in a community can force smaller mom-and-pop shops out of business because they can't compete on that scale -- thereby leading to less walkable neighborhoods and more driving. And most people would agree that if you eat cheap processed food while walking less and driving more, you're likely to get fat.

Obviously more research would need to be done to get to the bottom of this. But what do you think? Does access to cheaper food make people eat more? Do supermarket chains push unhealthful processed foods rather than the fresh foods that are better for us? Or are we guilty of bad choices and nutritional ignorance?

The researchers stressed that Wal-Mart should not be demonized. But what should we reasonably expect from big chains such as Wal-Mart and McDonald's?

Just last week, Wal-Mart announced a commitment to selling more-healthful food. Jeff Reeves of InvestorPlace.com, posting at our sister blog Top Stocks, wrote:

The in-store plan involves taking unhealthy salts, saturated fats, trans fats and sugar out of Wal-Mart store-brand products under the Great Value and Sam's Choice labels. The retailer also has pledged to lower prices on fruits and vegetables.

Other food companies are expected to follow suit.

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